Modification of cellular proteins by the Small Ubiquitin-like Modifier SUMO is essential for eukaryotic nuclear processes and cell cycle progression in yeast. Unlike ubiquitin conjugation, which can target proteins for degradation by the proteosome, SUMO modification targets proteins for changes in activity and localization. SUMO is translated as a precursor protein that is processed by proteases to generate a mature form which is then activated via C-terminal adenylation and transferred to an intramolecular cysteine by the SUMO activating enzyme E1 to form a thioester bond between SUMO and E1. The E1-SUMO thioester is then transferred to a cysteine residue within the E2 conjugating protein to form a thioester bond between E2 and SUMO. While the E2-SUMO complex is competent to transfer SUMO to lysine residues on the protein target, E3 ligases facilitate this process by binding substrates and E2-SUMO or by activating the E2-SUMO complex for conjugation. SUMO modification can be reversed by proteases that liberate both substrate and SUMO for additional rounds of conjugation. Established functions for SUMO are growing and now include regulating nucleocytoplasmic transport, nuclear body metabolism, transcription, cellular localization, chromosome segregation, DNA repair and replication, and regulation of cell cycle progression in so much as Saccharomyces cerevisiae genes for SUMO, a protease, E1, and E2 are all required for cell cycle progression through G2-M. Taken together, these observations suggest that SUMO conjugation plays a central role in regulatory processes involved in eukaryotic nuclear metabolism and cell cycle control, processes that are of direct relevance to human health, cancer, and the mission of the NIH. This proposal seeks to address the functional and structural significance for components of the SUMO conjugation apparatus. We will utilize structural, biochemical and genetic techniques to establish the basis for 1) SUMO isoform recognition by SUMO enzymes, 2) interactions between E1, SUMO, and E2 3) activities catalyzed by SUMO E3s and factors that recognize SUMO via distinct SUMO binding domains, 4) the kinetic and structural basis for RING-type E3 SUMO ligases. In addition to contributing to our understanding of SUMO conjugation, a thorough structure-function analysis of the SUMO pathway will yield insights into conserved mechanisms utilized in other ubiquitin and ubiquitin-like conjugation pathways.